The in vitro differentiation of clonal mouse embryonal carcinoma cells will be characterized biochemically using single cell assays for alkaline phosphatase, plasminogen activator, alpha fetoprotein and H2b antigen. The stability of the undifferentiated state will be determined by measuring the number of clones of different independently isolated embryonal carcinoma cell lines producing differentiated progeny from 1-7 days after plating. The early differential drug toxicity or cellular adhesion properties. The nuclear proteins of undifferentiated embryonal carcinoma cells will be compared by gel electrophoresis to those of primary cultures of early differentiated cell populations and to established endodermal cell lines. The possible induction of mesodermal derivates by primative endoderm will be investigated by co-cultivation of undifferentiated embryonal carcinoma and either primary cultures of endodermal cellular populations or established endodermal cell lines.